At present, a thin wire (bonding wire) having a wire diameter of about 20 to 50 μm is used mainly as the bonding wire bonding electrodes on a semiconductor device with external terminals. In order to bond the bonding wire, the ultrasonic wave-combined thermocompression bonding system is generally used and, general-purpose bonding apparatuses or capillary jigs used by passing the bonding wire through the inside thereof to connect are used. The end of the wire is melted by heating using an arc heat input to form a ball by utilizing a surface tension. Then, this ball part is press-bonded onto the electrode of the semiconductor device heated at a temperature in a range from 150 to 300° C., and then the bonding wire is directly bonded to the external lead side by the ultrasonic compression bonding. As mentioned above, this system bonds the ball parts to the electrode on such an IC chip to wedge-bond the bonding wire on electrodes of a lead frame and a substrate, and is called a forward bonding system.
In recent years, as to a packaging structure of a semiconductor, in addition to the current QFP (Quad Flat Packaging) using a lead frame, new structures such as BGA (Ball Grid Array) and CSP (Chip Scale Packaging) using a substrate and a polyimide tape have been put to practical use. There is therefore a demand for the bonding wire having better looping, bonding ability and mass-productivity.
Materials which are the subjects to be bonded of the bonding wire have been diversified, and in addition to conventional Al alloys, Cu suitable for the thinner wiring has been put to practical use for wiring on a silicon substrate and materials for electrodes. Also, there are many cases where the lead frame is plated with Ag, Pd or the like, and, Cu wiring is provided on a resin substrate and tapes or the like, and a film of a noble metal element such as gold or its alloy is formed on the wiring. It is required to improve the bonding ability and the reliability of the bonding part of the bonding wire corresponding to such various subjects to be bonded.
As the raw materials of the bonding wire, a high-purity 4N type (purity>99.99 mass %) gold has been primarily used so far. However, it is desired to use other types of metal with lower material cost for the bonding wire because gold is expensive.
The bonding wire using copper as its raw material is developed because it needs to be lower in material cost, excellent in electroconductivity and improved in, for example, ball bonding and wedge bonding, and which is disclosed in Patent Document 1 or the like. The connection of the copper-based bonding wire is characterized by the feature that when the ball part of the bonding is formed with blowing a nitrogen gas or a hydrogen-containing nitrogen gas to the end of the bonding wire to inhibit oxidation. At present, a nitrogen gas containing 5% of hydrogen is generally used as an atmospheric gas when forming the ball of the copper-based bonding wire. Patent Document 2 discloses that a copper wire is connected with a copper or copper alloy lead frame in a 5% H2+N2 atmosphere. Also, non-Patent Document 1 reveals that the 5% H2+N2 gas is more desirable than an N2 gas in the formation of the ball of the copper bonding wire because the 5% H2+N2 gas can inhibit oxidation of the surface of the ball.
The performance of the copper bonding wire is deteriorated by oxidation of the surface of the wire, and therefore a shorter storage life is concerned. In light of this, Patent Document 3 proposes the bonding wire obtained by coating copper with a noble metal or an anticorrosive metal such as gold, silver, platinum, palladium, nickel, cobalt, chromium or titanium as a method of preventing oxidation of the surface of the copper bonding wire.
The practical use of this copper-based bonding wire has been started in applications such as a power ICs using a large diameter having a wire diameter of about 50 μm. Moreover, in applications such as LSIs using thinner wires having a wire diameter of 25 μm or less, though the evaluation of the wire has been started, the copper-based bonding wire has not been put to practical use.
It is necessary that this copper-based bonding wire be adapted to various packaging structures for a wide prevalence in the fields of LSIs. It is necessary that this copper-based bonding wire can be adapted not only to conventional monolayer structures including only one semiconductor device, but also to, for example, laminate structures (stacked Chip) in which the plurality of semiconductor devices are vertically stacked, and multi-chip structures in which the plurality of chips are horizontally arranged. In memory LSI applications such as flash memories, many laminate structures are used. Therefore, the number of chips is increased to a level of five or more stacks, or special laminate structures such as an overhang structure having a difficulty in wire bonding are used. This leads to demand stricter requirements for wire connection. The laminate structure needs a specific connection called a reverse bonding, and it is therefore desired to develop reverse bonding technologies using the copper-based bonding wire.
The reverse bonding connection is a method for wedge bonding the bonding wire to the electrode on the IC chip by bonding the ball part onto the electrode of the lead frame and the substrate, which has a reverse positional relationship with the bonding of the above mentioned forward bonding method. In the reverse bonding connection, it is difficult to wedge-bond the bonding wire directly to the electrodes made of an aluminum film on the IC chip and therefore, a bonding using a stud bump made to be interposed between the bonding wire and the electrode is adopted. The ball part formed at the end of the bonding wire is bonded onto the surface of the electrode (hereinafter, referred to as electrode/bump bonding), and then, the bonding wire is cut to form a stud bump. In the course of the subsequent reverse bonding, the ball part formed at the end of the wire is bonded onto the electrode on the lead or the substrate and a loop is formed while moving the capillary from the lead side to the chip side. Then, the bonding wire is wedge-bonded onto the stud bump (hereinafter, referred to as bump/wedge bonding). This wedge bonding to the bump is advantageous in many points such as bonding strength, mass-production bonding ability and reliability. In the reverse bonding, there is a risk of occurring defects than in the case of the forward bonding because two bonding operations, the electrode/bump bonding and the bump/wedge bonding are carried out.
In this specification, a connecting system in which the bonding wire is wedge-bonded to a bump is defined as a reverse bonding. In the cases where chips are directly connected with each other by the bonding wire in the multi-chip structure, the bump/wedge bonding is commonly utilized. Although the positional relationship of bonding is not necessarily in a reverse direction, this case also falls under the category of the reverse bonding.
Patent Document 3 discloses that a gold wire, aluminum wire or copper wire is used as a material for the bonding wire in a connecting method similar to the reverse bonding connection. The connection method disclosed in this document is slightly different from the ordinary reverse bonding connection, which has a technique for stitch-bonding the bonding wire (same as “wedge-bonding”) without forming the ball in the connection to the lead side to minimize the time required for the wire bonding. Although it is described in Claims that gold and copper can be used as the raw material for the wire, there is no description concerning a difference between gold and copper, or descriptions of examples. There has actually been no substantial report concerning practical use of the copper-based bonding wire in the reverse bonding so far.
At present, only the gold bonding wire is used in the mass-production. Because a gold/gold bonding structure is formed in the bump/wedge bonding using the gold bonding wire, it is easy to obtain better bonding ability and good bonding strength. However, there is a fear that, as is evident from the fact that no copper-based bonding wire has been used for the reverse bonding, it may be difficult to practically use the bump/wedge bonding using the copper-based bonding wire. In order to use the copper-based bonding wire in applications of memory LSIs and multi-tip packagings, it is desired to improve, for example, the mass-productivity, bonding strength and reliability of the reverse bonding.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 61-99645
Patent Document 2: Japanese Unexamined Patent Application Publication No. 63-24660
Patent Document 3: Japanese Unexamined Patent Application Publication No. 62-97360
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2005-86200